(i) Field of the Invention
The present invention relates to an imaging apparatus based upon nuclear magnetic resonance (hereinafter referred to as "NMR"), and more particularly to a method of separately measuring nonuniformity in a static magnetic field that deteriorates the picture quality and nonlinearity in the gradient magnetic fields, and to a method of perfectly and independently correcting the effects thereof.
(ii) Prior Art
In the imaging employing nuclear magnetic resonance phenomenon, signals from various portions of an object must be separated and discriminated depending upon the positions.
According to a method taught in U.S. Pat. No. 4,602,214, the separation and discrimination are effected relying upon the application of a read-out gradient magnetic field and a phase encoding gradient magnetic field.
In such an imaging, nonuniformity in the static magnetic field and nonlinearity in the gradient magnetic fields are main causes that deteriorate the picture quality.
The specification of U.S. patent application Ser. No. 600,159 that was filed on Apr. 13, 1984, now U.S. Pat. No. 4,672,320 and that was assigned to the same assignee as the present application, discloses a method which, when the distribution of nonuniformity in the static magnetic field and the distribution of deviation of gradient magnetic fields from a straight line have been known, corrects the signals that are measured relying upon these distributions and removes distortion from the image. Further, according to a method of measuring the distribution of nonuniformity in the static magnetic field by Maudsley et al., Journal of Physics E: Scientific Instrument, 17, 216, 1984, the distribution of nonuniformity in the static field can be found to a degree of picture element as a unit.
The inventors of the present application has already filed in Japan a patent application covering a method of finding nonlinearity in the gradient magnetic fields separately from the nonuniformity in the static magnetic field. This method has been disclosed in Japanese Patent Laid-Open No. 84550/1986. According to this method, the measurement is effected twice under read-out gradient magnetic fields of two dissimilar strengths using a phantom of which the shape has been known, in order to detect the deviation of the gradient magnetic fields from the straight lines based upon the distortion of the reproduced image. In practice, however, the amount of distortion in the reproduced image is so small relative to the amount of deviation of the gradient magnetic fields from the straight lines that it is difficult to detect the distribution of deviation maintaining high precision, and leaves a problem in that complex calculation is required for correction.